Lightweight composite materials are employed for major structures in aircraft on a regular basis. Fiber composites provide a significant improvement in specific strength and stiffness over conventional metal alloys. Better specific strength and stiffness translates into weight savings, which translates into fuel savings and lower operating costs. Additionally, composites do not corrode like aluminum, and they are more resistant to fatigue. However, composite structures in these aircraft do not readily conduct away the extreme electrical currents and electromagnetic forces generated by lightning strikes.
Structural fabrication using composites still requires fasteners for many assembly operations. Metallic fasteners are conductive creating electromagnetic effect (EME) design considerations for lightning strike and other EME issues. Fastener sparking modes must be designed for lightning conditions including Hot Particle Ejection (HPE) and arcing between an outer surface of the fastener head and other structure or fastener sleeves.
When lightning strikes a Carbon Fiber Reinforced Polymer (CFRP) structure with metallic fasteners installed, a significant portion of the current may pass into nearby structures through the fasteners. When the electrical energy passes between two surfaces, contact resistance heating may break down the materials and generate hot gas (or plasma) in the hole, or in the space in-between a fastener and an associated internally threaded fastener such as a nut or frangible collar.
Sleeved fasteners are currently employed in many composite structures as a combined solution to lightning protection and structural requirements providing an interference fit between the fastener and an associated hole in the composite joint. However, sleeves create additional weight in the structure, additional cost and time in assembly and may have additional failure modes.
It is therefore desirable to provide a fastening system suitable for composite structural applications which overcomes both FIFE and arcing while overcoming the drawbacks of the prior art fastener installation procedures providing an interference fit without requirements for sleeves.